Technical ceramic bodies are well-known in the state of the art. They display a number of properties which make them suitable for a range of different applications. More in particular, such properties are hardness, physical stability, extreme heat resistance and chemical inertness, among others.
In many applications technical ceramic bodies are provided as coloured products. Colouring of the technical ceramic bodies leads to products which combine the favourable properties of the technical ceramic bodies with the aesthetic property of a particular colour. Applications where the colour increases the value of the products, or in other words where aesthetic properties are relevant, are applications in connection with jewelry or wrist watches.
The impregnation of technical ceramic products in order to achieve a colouring of the product is described in DE 2012304. Coloured ceramic products are achieved according to the teaching of the document by impregnating molded parts with aqueous solutions of colourant heavy metal compounds. The molded parts are prepared from suitable ceramic materials such as clay and kaolin which are formed into the wanted shape and treated with heat in the range of 800 to 1400° C. No pigment is present in the molded parts and the colouring is entered in the phase of the impregnation with the aqueous solution.
In WO 00/15580 the impregnation of ceramics is described for materials which comprise a non-coloured metallic oxide of spinel- or rutile-structure (TiO2, SnO2, ZrO2 or ZrSiO) that serves as a host lattice to fix bivalent or trivalent metallic ions. The ions are introduced through impregnation of an aqueous solution and allow to colour the ceramic. No indication of conditions used for pre-sintering, impregnation and drying is given. All examples concern the impregnation of clay by Ti/Sb/Cr solutions and yield yellow-ocre colours.
Faulkner & Schwartz (in “High Performance Pigments”, Wiley-VCH Verlag GmbH, 2009) describe aluminate-based pigments as “the combination of cobalt and aluminum oxides in a spinel-type stoichiometry (AB2O4 with A: Co and B: Al) that yields the blue cobalt aluminate spinel”. The colour of the spinel may be changed through the addition of other metals, such as zinc, magnesium, titanium or lithium.
In WO 2011/120181 A1 the preparation of coloured alumina-based opaque ceramics is described. The addition of an oxide of a metal chosen from chromium, cobalt, nickel, manganese, vanadium, titanium and iron leads to a coloured product. The preferred product described in the document is a red coloured technical ceramic body prepared from alumina containing chromium oxide.